The monitoring and regulation of fluid flow is vital in many industrial processes, particularly where corrosive fluids are utilized in the manufacture of electronic products such as integrated circuit devices. As an example, the flow of acid across a wafer for etching purposes regulates the depth of the etch and resulting quality of the devices produced.
In view of the corrosive nature of the chemicals employed in etching integrated circuit devices, equipment for handling such liquids must be protected against corrosion. It is also desirable that the liquid be isolated from the associated monitoring elements that measure fluid flow.
The above process requirements have led to development of the present flow meter, which is relatively simple in construction and capable of providing extremely accurate flow readings. It utilizes a conventional metering tube, which is normally termed a rotameter. Rotameters are exceptionally effective for measuring small flow values and are readily available at a relatively low cost.
In this type of flow meter, the weight of a float is balanced against upward forces imparted upon it by a moving fluid. Changes in flow rate result in changes in the upward force, with the float seeking an equilibrium position in a surrounding tapered metering tube. Although rotameters are very often made of glass, they can be constructed of other materials transparent to various forms of radiation. When coupled with radiation detectors, measurements of flow can be achieved by monitoring and measuring the elevation of the float without access to the liquid. Thus, a corrosive fluid can be isolated from an electronic flow detection system adjacent to the exterior of the tube.
The present invention was developed in an effort to improve upon previous detection systems for rotameters by eliminating precise calibration requirements, while decreasing the of incremental measurements that can be efficiently handled.